As technologies evolve, complementary metal-oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charged-coupled devices (CCDs) due to certain advantages inherent in the CMOS image sensors. In particular, a CMOS image sensor may have a high image acquisition rate, a lower operating voltage, lower power consumption, and higher noise immunity. In addition, CMOS image sensors may be fabricated on the same high volume wafer processing lines as logic and memory devices. As a result, a CMOS image chip may comprise both image sensors and all the necessary logic such as amplifiers, A/D converters and the like.
CMOS image sensors are typically pixelated metal oxide semiconductors. A CMOS image sensor typically comprises an array of light sensitive picture elements (pixels), each of which may include transistors (switching transistor and reset transistor), capacitors, and a photo-sensitive element (e.g., a photo-diode). A CMOS image sensor utilizes light-sensitive CMOS circuitry to convert photons into electrons. The light-sensitive CMOS circuitry typically comprises a photo-diode formed in a silicon substrate. As the photo-diode is exposed to light, an electrical charge is induced in the photo-diode. Each pixel may generate electrons proportional to the amount of light that falls on the pixel when light is incident on the pixel from a subject scene. Furthermore, the electrons are converted into a voltage signal in the pixel and further transformed into a digital signal by means of an A/D converter. A plurality of periphery circuits may receive the digital signals and process them to display an image of the subject scene.
A CMOS image sensor may comprise a plurality of additional layers such as dielectric layers and interconnect metal layers formed on top of the substrate, wherein the interconnect layers are used to couple the photo diode with peripheral circuitry. The side having additional layers of the CMOS image sensor is commonly referred to as a front side, while the side having the substrate is referred to as a back side. Depending on the light path difference, CMOS image sensors can be further divided into two major categories, namely front-side illuminated (FSI) image sensors and back-side illuminated (BSI) image sensors.
In a FSI image sensor, light from the subject scene is incident on the front side of the CMOS image sensor, passes through dielectric layers and interconnect layers, and finally falls on the photo diode. In a BSI image sensor, light is incident on the back side of the CMOS image sensor. As a result, light can strike the photo diode through a direct path.